1. Field of the Invention
The present invention relates to an improved automobile antenna system for efficiently detecting radio waves received at the vehicle body and transmitting detected signals to various built-in receivers.
2. Description of the Prior Art
Antenna systems are essential for modern automobiles to positively receive various waves such as radio waves, TV waves, car-telephone waves and others at built-in receivers in the vehicle bodies. Antenna systems also are very important for transmission and reception of waves in citizen band tranceivers.
There is generally known a pole type antenna which projects outwardly from the vehicle body. The pole type antenna exhibits a preferred performance receiving broadcast waves, but always provide an obstruction on the vehicle body design.
The pole type antenna also is subject to being damaged or stolen and further produces an unpleasant noise when an automobile on which the pole type antenna is mounted runs at high speeds.
Recently, the range of bands to which broadcast or communication waves belong is being increased. In such an event, the number of antennas must be correspondingly increased. This counteracts the aesthetic concepts in automobile design and also raises a problem in that electrical interference between the antennas remarkably degrades performance.
Some attempts have been made to eliminate or conceal the pole type antenna. One such attempt is that an antenna wire is applied to the rear window glass in an automobile.
There has also been made a proposal in which an antenna system is adapted to detect surface currents induced on the vehicle body by radio waves. Although this proposal is apparently positive and efficient, experiments showed that it could not effectively be used.
One reason why surface currents induced on the vehicle body by radio waves could not efficiently be utilized in the prior art is that the level of the induced surface currents is not as high as expected. The prior art utilized surface currents induced on the roof panel of the vehicle body. Notwithstanding, the outputs of sufficient level to be utilized could not be detected.
A second reason is that a very increased amount of noise is included in the surface currents on the vehicle body. The noise mainly results from the operation of ignition and regulator systems in an engine. The noise cannot be eliminated unless the engine is stopped.
One proposal for overcoming such problems in the prior art is disclosed in Japanese Patent Publication No. Sho 53-22418 in which electrical insulation is formed on the vehicle body at a location on which surface currents concentrate. Surface currents between the opposite ends of the insulation are detected directly by a sensor. Although this proposal can detect practicable signals which are superior in S/N ratio, it requires a pick-up which must be mounted in a notch formed in a portion of the vehicle body. This is not acceptable in mass-production.
Another proposal is disclosed in Japanese Utility Model Publication No. Sho 53-34826 in which an antenna system includes a pick-up coil used to detect surface currents flowing on a pillar in the vehicle body. However, the pick-up coil must be located adjacent to the pillar in a direction perpendicular to the length thereof. Such an arrangement is not practical and further does not provide practicable antenna outputs.
In the prior art, moreover, the resonance frequency of the antenna itself is fixed. When reception is to be carried out over wider bands of frequency, therefore, a plurality of antenna units are required.
Furthermore, the prior art antenna system is increased in size with an associated impedance matching circuit and pre-amplifier also being enlarged. This limits the location at which the antenna system is desirably located on the vehicle body.